This Meta SQL Question Tricks Almost Everyone

Description:
Most candidates fail this real Meta SQL interview question - not because they can't write SQL, but because they reach for the wrong tool. In this video, I'll show you the exact trap interviewers set, why the "obvious" solution fails, and the clean window function approach that gets you past the first round.

???? THE PROBLEM: Calculate the average session duration from a raw Facebook web log. A session = the time between a page load event and the very next page exit for the same user. Sounds simple — until you see the messy, noisy event data.

???? Practice the question here: https://platform.stratascratch.com/coding/10352-users-by-avg-session-time?code_type=1&utm_source=youtube&utm_medium=click&utm_campaign=YT+meta+sql+question+tricks+everyone

⚠️ THE TRAP (what most candidates do): A self-join. Joining the table to itself multiple times to pair loads with exits. It's fragile, bug-prone, and screams "junior developer" to your interviewer. One wrong operator and you've got a cross-day bug. One missed NULL check and you've got duplicates.

✅ THE PRO SOLUTION: Two CTEs + the LAG window function.
• CTE 1 - Filter down to only page_load and page_exit events
• CTE 2 - Use LAG() partitioned by user_id, ordered by timestamp to peek at the previous row
• Final SELECT - Subtract timestamps and average only valid load→exit pairs

No joins. No fragile WHERE clauses. Clean, readable, and exactly what Meta engineers want to see.

In this video you'll learn:
✅ Why the self-join fails (and the subtle bugs it introduces)
✅ How to use LAG() to compare sequential rows in one clean pass
✅ How to structure CTEs to keep your solution readable and interview-ready
✅ The mindset shift that separates mid-level from senior SQL candidates

???? KEY CONCEPTS COVERED: SQL window functions (LAG), Common Table Expressions (CTEs), Sequential event pair matching, Why self-joins fail for time-series data, How to think about event-based data the way top tech companies do

If you're preparing for SQL interviews at Meta, Google, Amazon, or any data role at a top tech company, this pattern comes up constantly. Master it once and you'll recognize it everywhere.
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???? Video Timeline:

0:00 – Why most candidates fail this Meta SQL question
0:24 – The problem statement: average session duration
0:50 – The trap: why self-joins are the wrong approach
1:28 – The pro solution: 2-step window function strategy
2:00 – Walking through the Facebook web log data
2:43 – Writing CTE 1: isolating the event sequence
3:01 – Writing CTE 2: using LAG() as the secret weapon
3:23 – Final SELECT: calculating average session duration
3:52 – Key takeaway
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About StrataScratch:

StrataScratch (https://platform.stratascratch.com/coding?code_type=2&page_size=100&utm_source=youtube&utm_medium=click&utm_campaign=YT+meta+sql+question+tricks+everyone) is a platform that allows you to practice real data science interview questions. There are over 1000+ interview questions that cover coding (SQL and Python), statistics, probability, product sense, and business cases.

So, if you want more interview practice with real data science interview questions, visit https://platform.stratascratch.com/coding?code_type=2&page_size=100&utm_source=youtube&utm_medium=click&utm_campaign=YT+meta+sql+question+tricks+everyone. All questions are free and you can even execute SQL and Python code in the IDE. Still, if you want to check out the solutions from other users or from the StrataScratch team, you can use ss15 for a 15% discount on the premium plans.

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???? Contact Us: Got questions or feedback? Drop them in the comments or email us at team@stratascratch.com.
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